Optical distance measuring instruments which can determine a distance between the distance measuring instrument and a target object are known. To this end, the distance measuring instruments emit a light beam in the direction of the target object and detect light reflected by the object and returning in the direction of the distance measuring instrument.
The distance may be determined by means of a time-of-flight method. The measurement may in this case be carried out in the time domain or in the frequency domain. In the case of a time-domain measurement, for example, a short laser pulse may be emitted at a time tstart to the target object, and scattered or reflected there. A part of this measurement radiation travels, for example, via a reception optical unit to the optical receiver and reaches the latter at a time tstop. The distance d of the target object is calculated from the measured time of flight (tstop minus tstart) and the velocity of light c0.
In order to improve the measurement accuracy, background radiation may be taken into account. To this end, a reference measurement path internal to the instrument, with a known distance between the light source and the reception apparatus, may be used. For example, such referencing is known from US 2007/0 182 949.
Even when taking the background radiation into account, the measurement accuracy may be reduced, for example, by systematic measurement errors. The systematic measurement errors may, for example, be caused by temperature-dependent signal times of flight, in particular within the evaluation circuit integrated in the distance measuring instrument.